1) Field of the Invention
The present invention relates to a composition for coating having a gas barrier property containing an epoxy resin. The composition is applicable to wide industrial fields including coatings aimed corrosion prevention and good-looking facing and coated films having a gas barrier property to be used as packaging materials for food and medicine in which a high gas barrier property is required.
2) Prior Art
An epoxy resin is applicable to wide industrial fields including coatings aimed corrosion prevention good-looking facing and adhesives for civil engineering and construction since it has many superior characteristics such as adhesiveness, heat resistance, chemical resistance, electrical characteristic and mechanical characteristic to other resins.
Recently, among packaging materials aimed preservation of contents, plastic films and containers have come to be used mainly from the aspects of transparency, light weight and economy. As performances of a plastic film to be used as packages for food medicine and cosmetic, a barrier property against various gases, transparency, retort treatment resistance, impact resistance and a heat seal property are required. In order to maintain performances and properties of contents, a high barrier property against both oxygen and water vapor under conditions of a high humidity and post retort treatment has been particularly required.
Generally, although a gas barrier property of epoxy resin compositions to be used in the field of coating is better than that of urethane resins, acrylic resins and polyolefin resins, it does not reach to that of polyvinylidene chloride (hereinafter, “PVDC”) and polyvinyl alcohol (hereinafter, “PVA”) to be classified into a gas barrier material. Accordingly, when an epoxy resin coating is used, in order to depress permeation of corrosion elements, various contrivances, e.g., to enlarge a thickness of a coated film; to perform coating by overlapping other material and to use together with a filler have been performed.
On the other hand, regarding a composition for coating used an epoxy resin, a method for improving a gas barrier property against oxygen and carbon dioxide by increasing amine nitrogen content in the composition has been suggested (Japanese Patent Publications Nos. 7-91367 and 7-91368). However, since the above-mentioned composition for coating exhibits neither a remarkably high gas barrier property, nor a high gas barrier property under the condition of a high humidity, further improvement has been desired.
Japanese Publication (PCT) No. 9-511537 suggests a method for improving a gas barrier property further than the above-mentioned composition and improving a gas barrier property under the condition of a high humidity by using a composition for coating composed of modified polyamine in which a ratio of active amine hydrogen in polyamine to epoxy group in polyepoxide is at least 1.5/1 and said polyamine is starting polyamine and at least 50% of carbon atom thereof is aromatic. However, the above-mentioned composition for coating causes a problem that when coating is performed to metals and concretes for the purpose of rust prevention and corrosion prevention, inherent excellent performances of an epoxy resin such as adhesiveness, heat resistance, chemical resistance and electrical characteristics are not developed since a large amount of amine group having unreacted active amine hydrogen remains in the reaction products after coating.
Generally, a gas barrier property of a thermoplastic film is not so high. Thus, as means to provide a gas barrier property to a thermoplastic film, hitherto, a method for coating PVDC resin has been mainly used. However, the PVDC coated film to be prepared by this method has become problem in that there is a fear that a toxic gas such as dioxin is generated during combustion because it contains halogen atom, so that environmental destroy is caused.
As arts to replace it, ethylene-vinyl acetate copolymer saponified product (hereinafter, “EVOH resin”) film, PVA coating and inorganic vapor deposition film vapor deposited silica (SiO2) or alumina (Al2O3) on a flexible polymer film have been known. The EVOH resin film and PVA coated film causes problems that when they are exposed to moisture under a high humidity or subjected to a boiling treatment or a retort treatment, its oxygen barrier property is remarkably deteriorated. Further, the inorganic vapor deposition film causes problems that crack or pin hole is generated in a gas barrier layer by bending since the gas barrier layer is formed by vapor deposition of a hard inorganic compound, so that its gas barrier property is remarkably deteriorated. Furthermore, such vacuum vapor deposition film necessitates a larger scale of production apparatus than coating or laminate film, so that it becomes a high price in the aspect of production cost.
Such packaging material having a gas barrier property is usually constituted by laminating each materials such as a flexible polymer film layer as a base material, a gas barrier layer and a flexible polymer film layer as sealant layer. Among them, as gas barrier materials to form a gas barrier layer, a PVDC coating and film, an EVOH film, a metaxylylene azipamide film, an inorganic vapor deposition film vapor deposited alumina or silica, a PVA coating and an epoxy coating increased amine nitrogen content in a composition (Japanese Patent Publication Nos. 7-91367 and 7-91368) have been known and applied by classification depending on species of contents and uses thereof based on their characteristics. However, such materials having a gas barrier property have not always good adhesiveness to various flexible polymer films. Thus, when a flexible polymer film as a sealant layer is laminated on a gas barrier layer, a dry laminating method for coating an adhesive on a gas barrier layer, thereby adhering a sealant layer and an extrusion laminating method for coating, optionally, an anchor coating agent on a gas barrier layer and adhering under an applied pressure a molten polymer layer as a sealant layer thereon, thereby laminating in a film form have been applied.
As an adhesive to be used in such methods, a two-liquid type polyurethane adhesive composed of a main gradient having an active hydrogen group(s) such as hydroxyl group and a curing agent having an isocyanate group(s) has been mainly used from the aspect of high adhesiveness, e.g., Japanese Patent Kokai (Laid-open) No. 9-316422. That is, in conventional laminate films for packaging material having a gas barrier property, in order to supplement adhesiveness between a gas barrier layer and a flexible polymer film layer as a sealant layer, it was necessary to provide another layer such as an adhesive layer and an anchor coat layer to perform a role of adhesiveness between the gas barrier layer and the flexible polymer film layer as a sealant layer, so that disadvantages were caused in the aspects economy and workability in the production steps.
Further, when laminating was performed with a two-liquid type polyurethane adhesive, it was necessary to perform post curing by aging after adhering in order to ensure sufficient adhesiveness. The aging time was as very long as 1 to 5 days since curing reaction of a polyurethane adhesive is generally so not rapid. When unreacted isocyanate group remained after curing, it was not preferable from the aspects of safety and hygiene, particularly in use of a packaging material for food. Further, there was a problem that the remained isocyanate group reacted moisture in the atmosphere to generate carbon monoxide, so that foams were generated in the interior of a film of a laminate layer.